Hitherto, a hydraulic pilot operated directional control valve has widely been used to control flow of hydraulic fluid supplied to hydraulic actuators in a hydraulic machine such as a hydraulic excavator or the like, to control motion of the hydraulic actuators.
FIG. 1 shows a hydraulic circuit having incorporated therein such hydraulic pilot operated directional control valve. In the figure, the reference numeral 1 denotes the hydraulic pilot operated directional control valve which is composed of a valve body section 2 and hydraulic pilot operating sections 3 and 4. The valve body section 2 is connected to a hydraulic pressure source 5 and a reservoir 6, and is connected to a hydraulic actuator 7, for controlling flow of hydraulic fluid supplied from the hydraulic pressure source 5 to the hydraulic actuator 7. The operating sections 3 and 4 are connected to a pilot valve 10 through respective pilot lines 8 and 9. The pilot valve 10 is adapted to generate a pilot pressure for controlling operation of the valve body section 2. The reference numeral 11 denotes a pilot hydraulic pressure source for the pilot valve 10.
Usually, the directional control valve 1 is formed into a multi-spool valve having accommodated therein a plurality of spools. The directional control valve 1 has connected thereto pilot valves and pilot lines corresponding in number to the spools. FIG. 1, however, shows only the hydraulic actuator 7 and a pilot circuit 8, 9, 10 with respect to one of the valve elements, for convenience of illustration.
As shown in FIG. 2, the directional control valve 1 has a plurality of spools 13 and 14 which are accommodated, for sealing movement, in a block 12 of the valve body section 2. The directional control valve 1 also has a plurality of pilot hydraulic pressure chambers 16 and 17 which are formed in a block 15 of the operating section 3 in a manner corresponding respectively to the plurality of spools 13 and 14.
Various grooves for controlling flow of the hydraulic fluid are formed in outer peripheral surfaces of the respective spools 13 and 14 and in opposed inner peripheral surfaces of the block 12. Of these grooves, FIG. 2 shows only drain grooves 18 through 21. The grooves 20 and 21 in the block 12 are connected to the reservoir 6 so that the hydraulic fluid is returned to the reservoir 6 through the grooves 20 and 21.
The pilot hydraulic pressure chambers 16 and 17 have accommodated respectively therein springs 22 and 23 for applying return forces respectively to the spools 13 and 14. Adapters 26 and 27 having therein respective pilot ports 24 and 25 are mounted to the block 15 through respective threadedly engaging portions 28 and 29. The pilot pressure is introduced into the pilot hydraulic pressure chambers 16 and 17 through the respective pilot ports 24 and 25. The adapter 26 is connected to the pilot valve 10 through the pilot line 8, while he adapter 27 is connected to a pilot line and a pilot valve which are not shown.
Although not shown, the hydraulic pilot operating section 4 is constructed in a manner similar to the operating section 3.
In this hydraulic pilot operated directional control valve 1, as the pilot valve 10 is operated, the pilot pressure is led from the pilot port 24 of the adapter 26 into the pilot hydraulic pressure chamber 16 through one of the pilot lines 8 and 9, for example, the pilot line 8, depending upon the direction in which the pilot valve 10 is operated. By this pilot pressure, the spool 13 is moved downwardly as viewed in the figure, to cause the hydraulic fluid from the hydraulic pressure source 5 to be supplied to the hydraulic actuator 7 through a plurality of grooves, not shown, formed in the spool 13 and the block 12, thereby driving the actuator 7. The returning hydraulic fluid from the hydraulic actuator 7 is returned to the reservoir 6 through the grooves 18 and 20.
By the way, in the hydraulic pilot operated directional control valve 1 constructed in this manner, air mingled in the pilot hydraulic fluid might be accumulated within the pilot hydraulic pressure chambers 16 and 17. If air is accumulated within the pilot hydraulic pressure chambers 16 and 17, switching speed, that is, responsibility of the hydraulic pilot operated directional control valve 1 with respect to the pilot pressure is deteriorated. Accordingly, in such case, so-called air bleeding is carried out in such a manner that the adapters 26 and 27 are detached from the block 15 to release air within the pilot hydraulic pressure chambers 16 and 17 to the outside.
However, operation of manually attaching and detaching the adapters 26 and 27 is considerably troublesome. In particular, it has taken time to remove oil adhering to the adapters 26 and 27 and the like. Moreover, even if the air bleeding has once been carried out, similar air bleeding has again been required to be effected after the elapse of a predetermined period of time, thus involving the maintenance cost.
It is therefore an object of the invention to provide a hydraulic pilot operated directional control valve in which release of air accumulated within a pilot hydraulic pressure chamber to the outside can be carried out automatically without the need of manual operation.